1. Field of the Invention
The present invention relates to a guide marker in which the reflecting direction of incident radio waves is not specified, and to a visual guide marker device used for restricting the range of travel of vehicles and for guiding the vehicles.
2. Description of the Related Art
In order to ensure safe driving of vehicles, such as automobiles, visual guide marker devices are placed on the roadside and median strips. The visual guide marker devices indicate the shape of a road and the centerline so as to restrict the range where vehicles can travel, and guide the vehicles in a predetermined direction so that the vehicles do not travel outside the range. As such visual guide marker devices, delineators, pole cones, and snow poles are widely known.
For example, a delineator includes a frame attached at the top of a post which is fixed to a guard rail and a light reflector having a reflection characteristic mounted to the frame. Also, a light-generating visual guide marker device is known, which includes a light-generating component.
In this type of visual guide marker device, when light from a light source, such as a headlight, is incident upon the light reflector, the light is reflected by the light reflector in the same direction as the incident direction so as to shine brightly. Accordingly, the reflected light is visually recognized by a driver such that visual guiding is performed. Under low visibility, for example, at night, in thick fog, or during snowfall, visual guiding is performed by light-generating components which generate light by themselves.
On the other hand, in recent years, Intelligent Transport Systems (ITS) has been used to improve convenience and safety in traffic so as to realize high efficiency. As part of the ITS, Advanced Cruise-assist Highway Systems (AHS) have been considered. In AHS, radio waves having a wavelength of a few millimeters or more are radiated from a radio-wave radar mounted on a vehicle, and the radio waves are reflected at a guide marker serving as a radio-wave reflector which is provided on a road and are received by the radar, such that the range where the vehicle can travel is restricted and the existence of obstacles is detected so as to guide the vehicle in an appropriate direction.
The AHS performs visual guiding for a driver. In addition, the AHS restricts the range where the vehicle can travel based on information generated by transmission/reception of radio waves using a vehicle-mounted radar so as to guide the vehicle, and attracts attention of the driver when the vehicle approaches a dangerous location.
A guide marker including a radio-wave reflector is disclosed in Japanese Unexamined Patent Application Publication No. 10-107540. In this guide marker, radiating elements which resonate at the frequency of incident radio waves from a specific direction and which reflect the radio waves in the incident direction are placed on the surface of the guide marker. That is, the guide marker is configured with the assumption that radio waves will enter the guide marker from a specific direction and that the radio waves are reflected in the specific incident direction. Specifically, a plurality of radiating elements including a conductive material, such as metallic foil, are printed in an array on an insulator, such as paint.
Also, Japanese Unexamined Patent Application Publication No. 10-107540 discloses radio-wave reflectors including radiating elements having concave and convex portions functioning as a square-cone corner reflector. In the radio-wave reflectors, a plurality of concave and convex portions are arranged in parallel on the surface of an insulator, the concave and convex portions are not smooth with respect to the wavelength of radio waves. In this guide marker, radio waves entering from a specific direction are reflected in the incident direction.
In order to establish general AHS as a system for detecting obstacles by using vehicle-mounted radars, various obstacles on a road or at the roadside must be reliably detected. That is, in order to realize this type of AHS, various signs and objects, such as direction signs, sound barriers, guard rails, and curbs, must be reliably detected as obstacles.
However, if an obstacle does not reflect radio waves, the obstacle cannot be detected by a radio-wave radar, and thus the existence thereof cannot be detected. Accordingly, in order to establish general AHS, signs and objects on a road or at the roadside must reflect radio waves.
In addition, since the positions and sizes of the objects are different from each other, it is insufficient that radio waves coming from a specific direction are reflected, and radio waves from various unspecified directions must be reflected. Despite such a requirement, the above-described guide marker only reflects radio waves from a specific direction in the same specific direction.
That is, this guide marker does not reflect radio waves from various directions in all directions in a virtual plane including the incident direction. Therefore, even when the known guide marker is attached to objects on a road or at the roadside, those objects cannot be detected using a radio-wave radar.
Under this circumstance, a known corner reflector, for example, a metallic triangular-cone-shape corner reflector is attached to the objects so as to define a guide marker. With this configuration, the corner reflector is more preferable than a metallic flat plate, which reflects radio waves coming straight from the front in that same direction, because the corner reflector can also reflect radio waves coming from a direction other than the front direction.
However, the corner reflector cannot reflect radio waves coming from a direction which is about 30° or more from the front direction. On the other hand, in the AHS, in which obstacles must be detected by a vehicle-mounted radar, a guide marker for reflecting radio waves coming from various directions, including a direction which is substantially orthogonal to the front direction, at a certain intensity is required. Clearly, the known corner reflector cannot satisfy this requirement.
Furthermore, as described above, known visual guide marker devices only perform visual guiding of a vehicle by indicating light which has been reflected at a light reflector to a driver. Under present circumstances, this visual guide marker device cannot restrict a range where vehicles can travel based on transmission/reception of radio waves by a vehicle-mounted radar and cannot notify a driver of the existence of a dangerous location.
Accordingly, even when visual guide marker devices including light reflectors having a reflection characteristic are installed on roads, if it is difficult for a driver to visually recognize reflected light due to the driver's carelessness or bad weather, dangerous accidents, for example, a vehicle running off the road and crashing into an obstacle, cannot be prevented. In particular, during snowfall, snow adheres to the visual guide marker devices so that reflected light and generated light are blocked. As a result, effects of visual guiding are significantly decreased.